1. Field of the Invention
The invention relates to separations conducted in the gas or vapour phase, and particularly to concentration of a desired component as a relatively purified product. In the practice of this invention, it will be desired to concentrate and purify the product with high efficiency, but it will not usually be required to achieve the highest possible recovery of the desired component from the feed stream. The invention applies for example to separation of oxygen or nitrogen from atmospheric air; or to purging of an impurity from a chemical process loop.
2. Prior Art
Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed, and is reduced during alternating intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds with directional valving to control the flow of compressed feed gas over each bed in alternating sequence, while the other bed is purged at low pressure by the reverse flow of a portion of the product gas. This conventional process makes inefficient use of applied compression energy, because of irreversible expansion over the valves while switching beds between higher and lower pressures. Also, the bed is closed at one end while the pressure is changing in response to flow into or out of the other end, and there will be flow at all points of the bed except the closed end owing to gas compressibility and changing adsorbent uptake as the pressure changes. This flow during pressure changing intervals can be detrimental to separation performance.
Some prior inventors have disclosed single bed pressure swing adsorption devices using mechanical cyclic volume displacement means such as pistons to generate cyclic flow and pressure variations in the bed. Examples of such devices with a piston at only one end of the bed include (Broughton) U.S. Pat. No. 3,121,625, (Wilson) U.S. Pat. No. 3,164,454, (Rutan) U.S. Pat. No. 3,236,028, and (Eriksson) U.S. Pat. No. 4,169,715. In each of these devices, the single piston is connected to the feed end of the adsorbent bed; and the resulting cycle will have inferior separation performance owing to lack of provision for a well defined purge step at the lowest pressure of the cycle.
Gardner (U.S. Pat. No. 4,207,084) has disclosed a single bed pressure swing adsorption device in which the adsorbent bed is mounted within a moving piston, with valving so that one side of the piston acts as a compressor. After completing the high pressure portion of its cycle, this device releases its internal pressure over a valve so that expansion energy is dissipated. This invention also has no provision for adequate purge flow when cycle pressure is minimum.
Keller (U.S. Pat. No. 4,354,859) has disclosed and tested a single bed pressure swing adsorption device with mechanical volume displacement means at both ends of the bed, with a specified range of phase angles between the two volume displacement means which are required to have unequal displacement. The cyclic flow and pressure regime is generated entirely by cyclic reciprocation of the volume displacement means at each end of the bed. The feed gas mixture is introduced to an intermediate point between the ends of the adsorbent bed, and a product enriched in the more readily adsorbed component is withdrawn from one end while a product enriched in the less readily adsorbed component is withdrawn from the other end. The Keller device can be effective in approaching substantially complete separation of a two component mixture, so that each component is concentrated into a product stream with high purity and high recovery simultaneously. It will be less effective for applications requiring concentration of only one component and not requiring high recovery of that component, as is often the case in air separation to generate either oxygen or nitrogen as a single product, because effort expended to concentrate the undesired second component will detract from the attainable purity and productivity of the desired product. Operation at high recovery will be inappropriate when the feed gas contains condensible components (such as water vapour in ambient air) which can deactivate the adsorbent (such as molecular sieves used for air separation).
A generalized class of thermally coupled pressure swing adsorption devices is disclosed in my U.S. Pat. No. 4,702,903. These devices use cyclic volume displacement means at each end of the adsorbent bed to generate the cyclic flow regime, and in general have a temperature gradient between the ends of the bed so that the cyclic pressure regime is determined both by the volume changes and by displacement of the gas between zones of different temperature. Gas separation is then combined with thermal energy conversion according to a regenerative thermodynamic cycle related to the Stirling or Ericsson cycles. The gases being separated may be chemically reactive within the apparatus. No means are provided for large exchanges of fresh feed gas for depleted feed gas during each cycle, so these devices may also have difficulties with adsorbent deactivation when the feed gas contains condensible components.
The present application is a continuation of my copending U.S. patent application 07/112,111and is related to my copending U.S. patent application 06/929,438. The latter patent application is concerned with pressure swing adsorption devices whose adsorbent bed has cyclically varied geometry, so that the gas volume along the flow path through the bed cyclically expands and contracts, in order to compensate for the compressibility of the gas and thus minimize flow while the pressure is changing. In most embodiments of the present invention, the adsorbent bed has a fixed geometry.